Presented below is background information on certain aspects of the present invention as they may relate to technical features referred to in the detailed description, but not necessarily described in detail. The discussion below should not be construed as an admission as to the relevance of the information to the claimed invention or the prior art effect of the material described.
The search for a membrane for use in gas separation applications that combines high selectivity with high flux continues. Current perfluoropolymer membranes, such as Hyflon® AD (Solvay), Teflon® AF (Du Pont), Cytop® (Asahi Glass), and variants thereof, have excellent chemical resistance and stability. We reported earlier, in U.S. Pat. No. 6,361,583, membranes that are made from glassy polymers or copolymers, including Hyflon® AD, and are characterized by having repeating units of a fluorinated, cyclic structure. In general, the ring structures in these materials frustrate polymer chain packing yielding amorphous polymers with relatively high gas permeability. These developed membranes are also more resistant to plasticization by hydrocarbons than prior art membranes and are able to recover from accidental exposure to liquid hydrocarbons.
It is known that copolymerization of fluorinated cyclic monomers with tetrafluoroethylene (TFE) enhances the chemical resistance and physical rigidity of membranes. TFE is also known to improve processability and has the effect of lowering gas permeability and increasing size selectivity in Hyflon® AD and Teflon® AF. Therefore, combinations of TFE with other monomer units, in particular perfluorinated dioxoles, such as Teflon® AF and Hyflon® AD, that result in overall amorphous, yet rigid, highly fluorinated, copolymers are preferred for industrial membrane applications. However, a drawback to these membranes is that their selectivities are relatively low for a number of gas pairs of interest, including H2/CH4, He/CH4, CO2/CH4, and N2/CH4.
Other than the commercially available perfluoropolymers, there is very limited gas transport data available for fully fluorinated polymers. Paul and Chio, “Gas permeation in a dry Nafion membrane,” Industrial & Engineering Chemistry Research, 27, 2161-2164 (1988), examined gas transport in dry Nafion® (an ionic copolymer of TFE and sulfonated perfluorovinyl ether) and found relatively high permeabilities and selectivities for several gas pairs (He/CH4, He/H2, and N2/CH4) compared to conventional hydrocarbon-based polymers considered for membrane applications. Nafion® and related ionic materials are used to make ion exchange membranes for electrochemical cells and the like. Because of their high cost and need for carefully controlled operating conditions, such as adjusting the relative humidity of the feed gas to prevent polymer swelling and loss of performance, these ionic membranes are not suitable for industrial gas separations.
Despite the improvements described above, there remains a need for better gas separation membranes, and specifically for improved membranes combining high flux, high selectivity, and good chemical resistance.
Recently, there have been reports of a new class of non-ionic amorphous perfluoropolymers. U.S. Pat. Nos. 7,582,714; 7,635,780; 7,754,901; and 8,168,808, all to Yoshiyuki Okamoto, disclose compositions and processes for making perfluoro-2-methylene-1,3-dioxolane derivatives.
Yang et al., “Novel Amorphous Perfluorocopolymeric System: Copolymers of Perfluoro-2-methylene-1,3-dioxolane Derivatives,” Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 1613-1618 (2006), and Okamoto et al., “Synthesis and properties of amorphous perfluorinated polymers,” Chemistry Today, vol. 27, n. 4, pp. 46-48 (July-August 2009), disclose the copolymerization of two dioxolane derivatives, perfluorotetrahydro-2-methylene-furo[3,4,-d][1,3]dioxolane and perfluoro-2-methylene-4-methoxymethyl-1,3-dioxolane. The copolymers were found to be thermally stable, have low refractive indices, and high optical transparency from UV to near-infrared, making them ideal candidates for use in optical and electrical materials.
U.S. Pat. No. 3,308,107, to Du Pont, discloses a similar dioxolane derivative, perfluoro-2-methylene-4-methyl-1,3-dioxolane. Homopolymers and copolymers of perfluoro-2-methylene-4-methyl-1,3-dioxolane with TFE are also disclosed.
U.S. Pat. No. 5,051,114, also to Du Pont, discloses the testing of poly-[perfluoro-2-methylene-4-methyl-1,3-dioxolane] for use in a membrane for gas separation. The results indicated that this material exhibited gas permeabilities 2.5 to 40 times lower as compared to dipolymer membranes of perfluoro-2,2-dimethyl-1,3-dioxole and TFE, but had higher selectivities.
To date, however, there have been no studies using copolymers of the perfluoropolymers described by Yang et al. and Okamoto et al. in membranes for gas separation processes.